Currently, environmental contamination in a soil, water and the atmosphere becomes a problem at the global scale.
Among the environmental contaminant, particularly, contaminant of a soil with heavy metals is serious. When checking the actual state of soil contaminant by heavy metal contained, contamination with lead is the highest as 29%, and contamination with arsenic is 24%, and contamination with hexavalent chromium is 15% in that order (The Ministry of the Environment “Summary of Examination Results Regarding Soil Contamination Examination-Subject Cases and Circumstances of Response Thereto” in 1999, March, 2001.
The soil lead contamination is reported in an old factory site, a firing range, a military exercise place or the like, and there is also a report of soil lead contamination around a road with lead released in the atmosphere by combustion of a tetraethyllead-containing gasoline (high-octane gasoline) which has been used until recent year.
On the other hand, a concern about the soil contamination has been increased, and a variety of techniques for cleaning up a soil contaminated with heavy metals or the like have been developed. For example, one is a soil washing method of mechanically washing a contaminated soil to remove harmful substances, and another, a heat-treating method of desorbing, degrading or dissolving contaminating substances by heating a contaminated soil to enclose the contaminating substances. These physical treating methods have an advantage that a term required for cleaning is short, but has a problem due to high cost, and heavy load to a soil.
To the contrary, as the low cost and the light environmental load technique, bioremediation (environmental cleaning utilizing organisms) and phytoremediation (environmental cleaning utilizing plants) technique, utilizing a nature that microorganisms or plants degrade or absorb harmful substances has been developed.
Phytoremediation is a soil cleaning method of absorbing heavy metals into a plant which has been seeded or planted and, thereafter, recovering heavy metals by harvesting the plant (reaping) and performing post-treatment (drying-degradation etc.). The method has an advantage that soil cleaning at the low cost is possible, and a load given to the environment is small since after a plant is seeded (planted) into a contaminated soil, labor hours until harvesting is saved. For this reason, utilization in extensive cleaning of contamination, prevention of contamination of a soil around a waste disposal site and the like are expected, and the following various publications are known.
Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-288529 (specification, paragraph (0005) etc.)
Patent Literature 2: JP-A No. 2002-336837 (specification, paragraph (0009) etc.)
Patent Literature 3: JP-A No. 2002-355665 (claim 5, paragraph (0018) etc.)
Patent Literature 4: Japanese Patent Application National Publication (Laid-Open) No. 2002-540943 (claims 44, 46 etc.)
Patent Literature 5: JP-A No. 2003-275741 (specification, paragraph (0007) etc.)
For example, in the known publication 2, a method effectively cleaning up a soil by selecting a plant excellent in the ability to absorb heavy metals, planting it into a contaminated soil is introduced, and, in the known publication 3, a method of adding a chelating agent to a soil to promote dissolution of heavy metals is introduced.
However, as a trend of a plant excellent in the ability to absorb heavy metals, a growth amount is small, and a growth speed is slow in many cases. The soil cleaning method by phytoremediation has a problem that since a necessary term of works depends on a growth speed of a plant, the term becomes longer. Further, the growing environment is limited in many cases, and a region of a contaminated soil where a plant can be cultivated is also limited.
In addition, in the method of adding said chelating agent to a soil, when the chelating agent is persistent (persistent with microorganisms or enzymes in the natural environment), the chelating agent which has not been absorbed by a plant remains in a soil. A fear that not only a plant itself undergoes a growth disorder with a remaining chelating agent, but also heavy metals fixed in a soil continues to be dissolved out, and is permeated deep in the ground, spreading secondary contamination, is pointed out.
In order to overcome such the problems, a soil cleaning method of specifying a plant to be used for cleaning, and utilizing a chelating agent selected from a group including biodegradable chelating agents (known publication 4) and a soil cleaning method utilizing L-glutamic acid diacetate (one kind of biodegradable chelating agents) (known publication 5) have been introduced.
However, a soil cleaning method which has a small fear of generating an environmental problem, and has a better efficiency of absorbing a large amount of heavy metals by a plant has not been reported.